A conventional system for industrial and commercial refrigeration or air conditioning might employ ammonia, for example, as a refrigerant. The ammonia, in gaseous form, is compressed in a compressor, from which it is discharged at a higher temperature and pressure. The compressed refrigerant gas travels to a condenser where it is liquified at a lower temperature. Cooled liquid refrigerant then travels through evaporator coils where it performs its cooling or refrigeration function by removing heat from the surrounding environment through the coils.
The evaporator coils normally accumulate moisture and, accordingly, frost during operation. Periodically these evaporator coils have to be defrosted in order to maintain the efficiency of the system. There are four widely used methods of defrosting evaporator coils. These might be characterized as the air method, the water method, the electric method and the hot gas method.
The hot gas defrost method is the most popular of the four. In the hot gas defrost method the supply of liquid refrigerant to the evaporator coil is interrupted and high pressure refrigerant vapor is delivered to the evaporator. While the high pressure refrigerant vapor is being delivered to the evaporator coil, the outlet of the coil is restricted so that a pressure is maintained in the coil. This provides a saturation temperature high enough to transfer heat to the frost or ice on the evaporator coils. As a result of this manipulation, the evaporator coil temporarily becomes a condenser coil. The latent heat given off into the frost during the condensation process is the major energy source for defrost.
To begin the defrost cycle, a first solenoid valve downstream of the condenser is closed and a second solenoid valve in a bypass line which leads directly from upstream of the condenser to upstream of the evaporator is opened. These solenoid valves normally open and close rapidly. When the bypass line has some liquid in it in addition to the hot gas from the compressor (as is frequently the case) a "slug" of liquid or a liquid-gas mixture rapidly passes through the second solenoid valve and strikes downstream system components, including the evaporator. What is known as "hydraulic shock" occurs and, particularly where the system is operating at low temperatures, severe damage to the system can result.